Spectroscopy

Spectroscopy is an optical technique in which UV, visible, or infrared radiation is passed through a sample. Filters isolate discrete bands of light that are absorbed by the specific component. Basically, a spectrophotometer consists of a light source, an optical filter, a flow cell, and a detector sensitive to a particular wave length. If only one component absorbs light in the wavelength region, a simple photometer provides accurate measurements. The amount of absorbed light is proportional to the concentration of each component. The technique is most useful where the concentration of a particular molecular group such as hydroxyls, paraffins, olefins, naphthenes, and aro-matics must be determined (see Figure 3.12.4).

Spectroscopy is faster and mechanically simpler than chromatography, and either direct-insertion or extractive sampling can be used. Spectroscopy is not precise when multicomponent mixtures are measured because a chemical's absorbances at individual wavelengths often interfere with each other. Multiple filters or a scanning instrument must be used in these solutions.

System computers are now equipped with data reduction programs called chemometrics. These programs compare sample spectra to known spectra stored in a database as a learning or training set. Establishing such a database requires examining and storing the spectral properties and reference methods for a significant number of calibration samples before the unit goes online.

For GCs controlled by external computers, the calibration requires only one sample, or six at most, to establish linearity in the expected concentration range. However, the chemometric models used by online spectrophotome-ters become more reliable as more samples are added to the learning set (Crandall 1993).